This all happened back when everything good was possible in America, and Life magazine was there to take pictures of it. It was reality then. It's in a museum now. "Life: The Second Decade, 1946-1955" is 200 terrific photographs at the National Museum of American Art.

"We thought that Life ran the world," said former assistant managing editor Maitland Edey on a recent television show. Life's photographers "were the bosses," Walter Cronkite said on the same show.

And Henry Luce himself, the founding father, once stood on a stage with Ed Sullivan and said: "We believe life has a purpose and our job is to show individual people and nations working out that purpose."

That's what Life magazine did. No publication or network before or since has had such power to tell Americans who they are.

The amazing thing, nowadays, is that it was all done with photographs that just sat there on the page with no sound track, no color, no stars of the magnitude of Cronkite to tell us what we were seeing.

Furthermore, so many of the pictures seem conspicuously posed -- a family relaxing in their new bomb shelter, Marilyn Monroe leaning heavy-lidded against a wall as her dress sidles off her shoulders, gangster Mickey Cohen seated amid scores of newspaper pages blazoned with his infamies, Adlai Stevenson sitting under an oak tree on his farm in Illinois "considering whether or not to seek the Democratic party's nomination."

In fact, even a lot of the action pictures seem posed: a couple of grizzled Marines advancing through the Korean mud, a teen-ager getting a haircut, an airplane flying supplies over people who watch from piles of rubble during the Berlin Airlift, or French troops parading in a final dress review before they evacuate Hanoi in 1954 -- the troops are mirrored in a huge puddle of water on the parade field, a picture of the world turned upside down.

They seem posed because they were made and edited with such craft, for one thing. Details are retouched to perfection -- better shadows and no nipples. The angles have been contemplated for days, and the equipment often installed by crews. Things were done right. In portrait after classic portrait you can see two lights reflected in each eye, one for the main light, one for the fill light. And these were the photographers Life had: Henri Cartier-Bresson, W. Eugene Smith, Margaret Bourke-White, Philippe Halsman, Gordon Parks, Gjon Mili, David Douglas Duncan and Alfred Eisenstaedt, among others.

The pictures seem posed, too, because they touched so many American stereotypes -- the politician/farmer, the country doctor, the wacky artist, the fad-bound teen. More important, they touched the stereotypes we had of ourselves. The picture of a Korean orphan waiting for a meal told us that yes, we are a people who love children. The old woman collecting leftover catsup after a bean supper on Boston Common shows us that we value thrift and pity the poor. The horrible truck that bristles with the feet of dead Marines reassures us that we hate war. And picture after picture of inept recruits or fat people at the beach tells us that we can be proud of having a sense of humor about ourselves.

Ultimately, they seem posed because they were advertisements for reality. Their purpose was to show that life had a purpose, as Luce was bold enough to say to the nation, and that we were good.

It was television that did Life in. Television delivered the pictures instantly, and satellites delivered television from anywhere in the world. Also, television and film have a way of making all authority look evil. A still photograph of a Vietnam Marine setting a grass hut on fire might reveal his fear or anguish. On television, he became a symbol of mindless American brutality. (The Nazis learned this lesson long before. A legendary propaganda film on the suppression of the Warsaw ghetto was never released because the Nazis discovered, it's said, that no amount of selected camera angles or editing could make them look good, even to their supporters.) If authority was evil, then Life magazine's America had vanished. A sly jab at that America is the first picture in the catalogue for this show. The picture shows an audience at the premiere of a 3-D movie, "Bwana Devil" -- all of them looking in the same direction, with the same expressions, and the same preposterous paper spectacles on their faces. Bwana Devil, indeed.

The weekly Life was founded in 1936. It died in 1972, but it was losing its cultural clout long before. By the end of the '50s, a ridiculing of the Time-Life Weltanschauung was part of the spiel of every hip young college professor. Countercultures of teen, beatnik and rock rose to challenge orthodoxies -- all covered by Life's photographers to a point that it's hard to imagine those movements gaining such power over our national imagination without them.

Finally our leaders in the '60s took one look at the wealth of meaning and optimism they'd inherited and spent it like wastrel sons.

The prints on view come from the magazine's files of 156,000 pictures. They're creased and spotted, a lot of them, but that adds authenticity. To see them you have to take enough turns around enough free-standing walls in the middle of the room that you start to lose track of where you are, but maybe that adds to the sense of discovery.

In any case, television gives you the picture, says Cronkite, but it doesn't give "you a chance to contemplate its meaning." Nobody ever said that about Life magazine.

The show stays on view through May 12. A companion exhibit lies across the courtyard in the National Portrait Gallery -- a collection of pictures of the 23 living children of presidents that appeared in the November 1984 Life. They are smiling, they are posed. And did you know that Grover Cleveland's son is alive and well in New Hampshire?